The Merrick physician facing federal charges of illegally prescribing opioids, and causing the overdose deaths of two South Shore men, began on Wednesday, with prosecutors calling Dr. Michael Belfiore “a dealer, not a healer,” and Belfiore's defense attorney insisting that the doctor is being unfairly prosecuted.

Belfiore's trial, at the U.S. District courthouse in Central Islip, is expected to last five weeks, according to his attorney, Tom Liotti, of Garden City.

After a jury was selected, Assistant U.S. Attorney Bradley King made his opening argument, describing the circumstances in which John Ubaghs, of Baldwin, and Edward Martin, of East Rockaway, were found dead — both allegedly with bottles of oxycodone prescribed to them by Belfiore.

King also introduced the government's first witness against Belfiore: Detective James Marinucci, of the Nassau County Police Department's vice squad. Marinucci — undercover as James Burke, a factory worker with back pain — saw Belfiore as a patient six times in 2013, obtaining six prescriptions for oxycodone. He paid in cash each time.

In a lengthy video — taken by a hidden camera Marinucci wore on a necklace — shown to jurors, the undercover detective was seen and heard during an initial appointment with Belfiore in March 2013.

During the visit, Marinucci complained of back pain, and told Belfiore that his ex-girlfriend used to share her “Oxy 30s” with him — Marinucci testified that he used this “street” phrase for the medication deliberately with Belfiore.

Belfiore agreed to write Marinucci multiple prescriptions, including for oxycodone, anti-inflammatories and Trazodone, for sleep, but did warn the undercover detective — using colorful language — about the dangers of sharing controlled substances with others.

“I’m not gonna share a jail cell with you,” Belfiore joked at one point, later stressing, “You don’t understand the stigma that’s attached to these medications now.”

Liotti was expected to cross-examine Marinucci on Thursday.

Liotti, has maintained that opioid manufacturers are the culprits in Ubaghs’s and Martin’s deaths — and in the country’s larger opioid crisis.

The defense attorney reiterated the point during his opening arguments, also calling the grand jury process that led to Belfiore’s indictment, in which he was not allowed to be present, a “one-sided proceeding.”

“We offered our own expert testimony — the government wouldn’t allow it,” Liotti said, also warning jurors that he believed the government would try to connect Belfiore’s case to the hundreds of thousands of opioid deaths nationwide.

He also challenged prosecutors to define the number of pills Belfiore could have prescribed that would have met their definition of “with a legitimate medical purpose.”

“There can be no guess-work or speculation here,” he added.

Belfiore also, Liotti said, had been honest with law enforcement throughout the yearslong case, “perhaps to a fault,” and made reference to both Arthur Miller’s “The Crucible,” and the film “12 Angry Men,” as he tried to paint Belfiore as the government’s scapegoat.

“His career and his life are on the line,” Liotti said.

Belfiore was also implicated in the 2009 death of Mario Marra, of Glen Cove. Medical records in the case were subpoenaed, although charges were not filed.

In a series of Herald reports last summer, Marra’s widow, Claudia, alleged that Belfore continued to prescribe her late husband fentanyl and other opioids, even after he knew Marra was addicted.

Medical records indicate that Belfiore prescribed Marra fentanyl on March 7, 2009. He died on March 15, according to the coroner’s report.

Belfiore, in an interview last summer, admitted Marra was a patient, but disputed much of Claudia’s account, adding that if Marra was “responsible with the medication, and took it as directed, he’d still be here.”

Look for more coverage of Belfiore's trial in next week's edition, and online.